This invention relates generally to differential amplifiers, and in particular to a method for correcting common mode errors in differential amplifiers.
Differential amplifiers am well known in the art, and have been used for many years to measure the difference between two signals or voltages applied to the two input terminals thereof. A customary specification of differential amplifiers is known as common-mode rejection ratio, which is a measure of the ability of the differential amplifier to block common-mode components of signals or voltages while amplifying the differential signals or voltages.
FIG. 1 shows a typical prior art differential amplifier circuit in which an adjustable component is used to balance the two sides of the amplifier and thereby correct common mode error. An amplifier 10, which may commonly be an integrated circuit, has a pair of inputs, labeled + and - respectively, and an output. The + input is connected to the junction of a pair of resistors 12 and 14 disposed in series between a first input terminal 16 and ground, while the - input is connected to the junction of a pair of resistors 18 and 20 disposed in series between a second input terminal 22 and an output terminal 24, to which the output of the amplifier 10 is also connected. In this configuration, the value resistor 12 is ideally equal to the value of resistor 18, and the value of resistor 14 is ideally equal to the value of resistor 20, thus ideally balancing the two sides of the amplifier. In practice, however, the resistance values are not exactly equal, and therefore one of the resistors, for example, resistor 12, is made variable. The circuit is calibrated by shorting terminals 16 and 22 together and applying a common mode voltage or signal thereto from a calibration source 28 while monitoring the output with a voltmeter 30 connected between output terminal 24 and ground. Resistor 12 is adjusted to provide a reading of zero volts on the voltmeter 30, indicating complete rejection of the common-mode signal or voltage. With the two halves of the amplifier thus balanced, calibration is complete and the differential amplifier may be operated as intended. For extremely high precision differential amplifier action, such as desired for instrumentation amplifiers, it is very difficult to correct for common mode error with balance adjustments unless very expensive linear potentiometers are used. The advent of surface-mounted resistors and laser trimming has permitted some precision to be built in; however, one of the problems of manual calibration or built-in precision is that component values drift with heat and age, and recalibration or replacement becomes necessary.